backdoor

adj.

also back-door, "devious, shady, illegal," 1640s. The notion is of business done out of public view. The noun back door in the literal sense is from 1520s, from back (adj.) + door. The association with sodomy is at least from 19c.; cf. also back-door man "a married woman's lover," black slang, early 20c.

back door

The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D.Copyright (C) 2007 by HarperCollins Publishers.Cite This Source

Idioms and Phrases with back-door

back door

1.

An entry at the rear of a building, as in Deliveries are supposed to be made at the back door only.
[ First half of 1500s
]

2.

A clandestine, unauthorized, or illegal way of operating. For example, Salesmen are constantly trying to push their products by offering special gifts through the back door. This term alludes to the fact that the back door cannot be seen from the front.
[ Late 1500s
]